Dynamics of dissolved organic carbon in eutrophic Lake Taihu and its tributaries and their implications for bacterial abundance during autumn and winter

Surface water samples of Lake Taihu and its tributaries were collected from September 2012 to February 2013 to investigate the effects of dissolved organic carbon (DOC) dynamics on bacterial abundance. DOC concentrations, stable carbon isotope values (delta C-13(DOC)), dissolved carbohydrates, and specific ultraviolet absorbance (SUVA(254)) were analyzed along with several environmental factors: water temperature, electrical conductivity (EC), chlorophyll a, and bacterial abundance. The highest DOC concentrations were observed in January 2013 at all sampling sites except at lake center. DOC concentrations decreased along both transects from the tributaries to the lake center in January 2013. In addition, the evidence from the positive correlation between DOC and EC in Lake Taihu suggests the input of river water into the lake. The variation in DOC concentration can be explained by temperature-dependent bacterial utilization of organic compounds. The delta C-13(DOC) values ranged from -26.9 parts per thousand to -23.4 parts per thousand in Lake Taihu and its tributaries, suggesting an influence of autochthonous sources and microbial utilization. There was no significant relationship between the ratio of total dissolved carbohydrates to dissolved organic carbon (TCHO/DOC) and bacterial abundance, indicating a complex bacterial metabolic effect on carbohydrates. The spectroscopic character of DOC shifted toward less humic material from autumn to winter and correlated positively with bacterial abundance, suggesting that humic substances might serve as a substrate for bacteria in Lake Taihu and its tributaries. These results demonstrate that DOC may originate from autochthonous and allochthonous sources and that the quantity and quality of DOC changes considerably during autumn and winter, with important effects on bacterial abundance.